These studies were undertaken to investigate the effects of selected environmental chemicals on the immune competence of laboratory animals. The topics of present interest include (1) effects of transplacental exposure; (2) effects of neonatal exposure; and (3) effects of adult exposure. Chemicals investigated to date are 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,4,5,3',4',5'-hexachlorobiphenyl (HCB), and methyl mercury. Results of studies completed to date show TCDD to profoundly suppress cell mediated immune reactions, HCB to suppress immune response to bacterial lipopolysacchamire, and methyl mercury not to have significant effects at the time periods tested.